1. Field of the Invention
The present invention relates to a surface light source device, illumination unit and light flux control member, being applied, for instance, to a surface light source device for backlighting a liquid crystal display panel, an illumination unit adopting the surface light source device as an illuminating means, or a light flux control member employed in those. More specifically, surface light source devices, illumination units and light flux control members in accordance with the present invention are able to be applied to backlighting arrangements for TV's or personal's computer monitor displays.
2. Related Art
A surface light source device employing a plurality of LEDs (light emitting diodes) as point-like light sources has been known as an illuminating means for a LCD monitor display of a personal computer or TV set. A plate-like light flux control member having roughly the same shape as that of a LCD panel is employed in the surface light source device, being provided with a plurality of LEDs arranged like a matrix at the back side. The LEDs emit light which is incident to a back face of the light flux control member and travels within the light flux control member to an emission face opposite to the back face, being outputted from the emission face toward a LCD panel to be backlighted. Prior arts like this have been disclosed in the following documents.
<Prior Art 1>
This is found disclosed in Tokkai 2002-49326 (JP-A) 2002-49326), according to which surface light source device 10 is provided with microlens array 102. Individual microlenses are arranged in one-to-one correspondence to a plurality of LEDs 101, as shown in FIG. 22. Light from LEDs 101 is outputted in a direction perpendicular to a plane (upward) via microlens array 102.
<Prior Art 2>
This is found disclosed in Tokkaisho 59-226381(JP-A 1984-226381), according to which emission display device 103 is provided with LED 104, concave lens 105 and convex lens 106, as shown in FIG. 23. Light from LED 104 is condensed by convex lens 106 after being diverged by concave lens 105, being outputted in a direction roughly parallel with an “optical axis” of LED 104. Please note that “optical axis” is defined as a light travelling direction at a center of three-dimensional light flux emitted from a point-like light source (LED 104).
<Prior Art 3>
This is found disclosed in Tokkaisho 63-6702 (JP-A 1988-6702), providing an illumination unit 107 having LED(s) 108, as shown in FIG. 24. Light from LED 108 is condensed by condenser lens 110 and directed forward, then being diverged by diverging lens 111.
<Prior Art 4>
Another prior art provides illumination unit 1 as shown in FIG. 17b. 
Illumination unit 1 is provided with light flux control member 4 and LED 5 which is arranged at the side of back face 4a of light flux control member 4. Back face 4a has a semi-spherical recess 60 facing to LED 5 so that light from LED 5 enters into light flux control member 4 via recess 60. The light is outputted from emission face 4b. 
However, the above prior arts involve problems as follows.
(1) Prior Art 1;
Surface light source device 100 has a portion at which configuration of microlens array 102 varies discontinuously. The portion is located between LEDs 101 adjacent to each other. Emission intensity changes sharply at this discontinuity portion, with the result that a conspicuous unevenness in brightness appears around boundary regions between individual microlenses of microlens array 102.
(2) Prior Art 2;
It is difficult to say that concave lenses 105 in emission display 103 are coupled with each other continuously to form a plane. Further to this, convex lenses 106 are scarcely coupled with each other continuously to form a plane. Therefore, a member of a large area to be illuminated, such as large-screen liquid crystal display panel, is hardly supplied with uniform illumination light.
(3) Prior Art 3;
With illumination unit 107, light from LED 108 is diverged by diverging lens 111 after being condensed by condenser lens 110. This will reduce unevenness in brightness as compared with Prior Art 1. However, a sufficient mixing of light fluxes from LEDs 108 adjacent to each other is hardly expected, with the result that unevenness in emission color between individual LEDs 108 tends to be conspicuous.
(4) Prior Art 4;
With illumination unit 1, emission angle θ1 at emission from LED 5 and emission angle θ5 at emission from light flux control member 4 satisfy a relation θ5/θ1>1 for light from LED 5 other than light within an angular neighbourhood of a normal direction as illustrated in FIGS. 17b and 4. Please note that “normal direction” is defined as a direction which accords with optical axis L in FIG. 17b and a normal perpendicular to emission face 4b of light flux control member 4. In other words, emission angel θ1 at emission from LED 2 is greater than emission angel θ5 at emission from light flux control member 4. This enables emission fluxes from LED 5 to be converted into expanded emission fluxes.
However, an incidence to an end edge of a recess 60 brings light that intersects with light brought by incidence to a neighbourhood of the end edge because the end edge of recess 60 (connecting portion between semi-sphere-like recess 60 and generally flat back face 4a) gives a sharp edge. This intersection of light raises a problem of ring-like emission. As a result, illumination unit 1 fails to expand light from LED 5 smoothly and effectively to a desirable range.